Study and development of a didactic engraving system using a low
powered laser diode
António Jorge Reis Lordelo Paulos
Dissertation Supervisors:
Prof. Manuel Rodrigues Quintas Eng.º Jorge Manuel de Matos Reis
Engineering Faculty of the Universit y of Porto Master’s Degree in Mechanical Engineering
iii Abstract
Laser engravers may be used in a didactic environment for the augmentation of certain activities. Whether it is for training involving motion control systems or for use as a tool in the context of other disciplines, a laser engraving and/or cutting machine is desired. Thus, the present dissertation reports about the study and development of such a system. The main objective is to develop a laser engraving system for didactic purposes. The engraver is meant to use a low powered diode laser device for engraving and/or cutting soft materials.
Firstly, a preliminary study of diode laser fundamentals, as well as laser engraving and cutting technologies, was carried out. This allowed acquiring basic knowledge about the device to be used and which sort of configurations are most common for the axes of motion of laser engraving and cutting machines. Furthermore, it served as a basis for the definition of a concept for a working prototype.
As such, the project specification ensued to establish the requirements and characteristics to meet in the development of the prototype. Safety, usability and maintenance issues were considered and the technical aspects that may characterise the prototype were addressed. Then, a model of the prototype was designed in order to study a solution for its creation. The design process encompasses a few iterations which have been discarded before the final version was deemed satisfying.
This final model has been complemented by the control system solution, implemented after studying an existing software suite capable of numerically controlling the axes of motion of the prototype. Also, a driver was considered for the diode laser device that was chosen as the laser beam source.
v Resumo
Num contexto didático poder-se-iam utilizar máquinas de gravação/corte a laser para o enriquecer de algumas atividades. Tanto para formação com sistemas de controlo de movimento como para uso como uma ferramenta no âmbito de outras disciplinas, houve o desejo de conseguir uma máquina de gravação/corte a laser. Assim, o presente trabalho relata o estudo e desenvolvimento de um sistema deste tipo. O principal objetivo é então desenvolver um sistema de gravação a laser para fins didáticos. Pretende-se ainda usar um dispositivo de laser díodo de baixa potência para a gravação e / ou corte de materiais macios.
Em primeiro lugar, um estudo preliminar dos conhecimentos fundamentais sobre díodos laser foi realizado, bem como sobre as tecnologias de processamento de materiais por gravação e corte a laser. Isso permitiu a aquisição de conhecimentos básicos sobre o dispositivo a ser usado e sobre máquinas de gravação e/ou corte a laser. Além disso, serviu de base para a definição de um conceito para um protótipo funcional.
Desta forma, prosseguiu-se com uma especificação do projeto de modo a estabelecer os requisitos e características do protótipo a desenvolver. Foram consideradas as devidas questões de segurança, usabilidade e manutenção, assim como se abordaram os aspetos técnicos que caracterizem o protótipo.
Em seguida, um modelo do protótipo foi concebido para estudar uma solução para a sua construção. O processo de criação desse modelo engloba algumas iterações que foram descartadas para que uma versão final fosse considerada satisfatória.
O modelo final foi complementado pela solução de sistema de controlo de movimento, implementada após o estudo de um software existente capaz de controlar numericamente os eixos de movimento do protótipo. Além disso, foi considerada uma solução para alimentação de um laser díodo, tendo um destes dispositivos sido escolhido como a fonte do feixe de laser para o protótipo.
vii Agradecimentos
Ao meu orientador, Professor Manuel Rodrigues Quintas, pela disponibilidade e pelo indispensável apoio e esclarecimento ao longo deste trabalho.
Ao meu coorientador, Engenheiro Jorge Manuel de Matos Reis, pela paciência e disposição com que sempre me recebeu, assim como pelo apoio prestado.
Ao Professor Francisco Jorge Teixeira de Freitas pelo acompanhamento e pelo interesse na minha atividade e de todos os mestrandos.
Ao Engenheiro Bruno Santos, ao Engenheiro Tiago Andrade e ao Engenheiro António Silva que me ajudaram bastante na realização deste trabalho.
Ao Sr. Joaquim Silva, técnico de laboratório, que muita paciência teve comigo também.
Aos meus pais que comigo muito sofreram e ainda assim mais valor me deram do que aquele que considero ter. Também à minha irmã que da mesma forma irá lidar com o derradeiro desafio no seu percurso académico.
À minha namorada e amigos sem os quais também não teria a força para persistir e que muito fizeram pela minha autoconfiança.
ix Table of Contents Abstract ... iii Resumo ... v Agradecimentos ... vii Table of Contents ... ix Table of Figures ... xi Table of Tables... xv 1. Introduction ... 1 1.1 Motive... 1 1.2 Objectives ... 2 1.3 Report structure ... 3 2. Preliminary study ... 5
2.1 Study of laser diodes ... 5
2.1.1 Laser fundamentals... 5
2.1.2 Diode laser devices ... 8
2.2 Laser processing systems ... 12
2.2.1 Laser processing of materials ... 12
2.2.2 Systems for laser cutting and engraving ... 14
3. Project specification ... 19
3.1 Requirements ... 19
3.1.1 Safety concerns ... 19
3.1.2 Usability, maintenance, and safety features ... 23
3.2 Prototype definition ... 25
3.2.1 Concept and design considerations ... 25
3.2.2 Technical characteristics ... 29
4. Model and mechanical assembly of the prototype ... 31
4.1 Initial modelling stages of the design ... 31
4.2 Final stage of the model ... 38
4.3 Mechanical assembly ... 41
4.3.1 Structural frame ... 41
4.3.2 Y axis ... 42
4.3.3 X axis ... 44
4.3.4 Z axis and assembled prototype ... 49
5. Motion control system and laser device ... 51
5.1 Motion control system ... 51
5.1.1 Driving system ... 51
5.1.2 Control software ... 59
5.1.3 Control system configuration and setup ... 65
5.1.4 Part program creation and execution ... 69
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6. Demonstrative working prototype ... 77
7. Conclusions and future work ... 81
References ... 83
ANNEX A: Description of laser classes ... 85
ANNEX B: TB6560 Excitation modes ... 87
ANNEX C: TB6560 Decay modes ... 91
ANNEX D: TB6560 Transistor operation ... 93
xi Table of Figures
Figure 1 - Example laser engravers, a) Epilog Zing desktop laser engraver; b) Epilog Fusion
laser engraver and cutter. ... 2
Figure 2 - Stimulated emission illustration diagram. ... 6
Figure 3 - Schematic of Theodore Maiman’s ruby laser. ... 6
Figure 4 - Simplified diagram of the elements of a laser device. ... 7
Figure 5 - a) TO-can diode laser package; b) cross section diagram of such a package. ... 8
Figure 6 - A simple laser diode homojunction structure. ... 8
Figure 7 - Layers forming a double heterostructure. ... 9
Figure 8 - Light output vs current graph indicating the laser, or lasing, threshold. ... 11
Figure 9 - Classification of some laser processing applications by phase change mechanisms. ... 13
Figure 10 - Broader classification of some laser processing applications. ... 13
Figure 11 – Laser processing machine components: 1-laser source; 2-laser beam delivery; 3-cutting head; 4-work table axis of motion; 5-control unit; 6-power supply unit. ... 14
Figure 12 - Fixed optics configuration schematic. ... 15
Figure 13 - Flying optics configuration schematic. ... 15
Figure 14 - Scanned projection configuration schematic. ... 16
Figure 15 - Warning label. ... 22
Figure 16 - Explanatory label. ... 22
Figure 17 - Class 2 laser product warning and explanatory labels. ... 23
Figure 18 - Protective housing sketch. ... 25
Figure 19 - a) hall-effect sensor example with working principle schematic; b) emergency stop button example. ... 26
Figure 20 - Hybrid gantry configuration sketch indicating the axes of motion. ... 27
Figure 21 - Mirror guiding schematic... 28
Figure 22 - First draft of the structural frame. ... 31
Figure 23 - Constraining dimensions sketch (top view of first draft). ... 32
Figure 24 - Misumi 5 series T-slot aluminium extrusions... 32
Figure 25 - Structural frame overall dimensions. ... 33
Figure 26 - Firstly proposed solution for the Y axis transmission. ... 34
Figure 27 - Earliest prototype model. ... 35
xii
Figure 29 - Final version of the prototype’s model. ... 38
Figure 30 - Back panel featuring a DB25 connector and power chassis for the electronic components inside. ... 39
Figure 31 - Emergency stop button, mounted on a printed part that fastens to the protective housing... 39
Figure 32 - Hall-effect switch and magnet, viewed from the inside of the structure (wiring not represented). ... 40
Figure 33 - Mechanical switch: a) represented in the model; b) image of the physical device. ... 40
Figure 34 - Node fastening solution. ... 41
Figure 35 - Final aspect of the assembled structure. ... 41
Figure 36 - Highlighted Y axis. ... 42
Figure 37 - Isolated underside view of the Y axis. ... 42
Figure 38 - a) transmission assembly; b) section view of assembled mechanism. ... 43
Figure 39 - The level adjustment screws position and side view detail. ... 43
Figure 40 - Highlighted X axis. ... 44
Figure 41 - Isolated view of the X axis. 1-X axis stepper mount; 2-X axis cart; 3-X axis mirror mount. ... 44
Figure 42 - Beam guiding, suface coated mirrors of the X axis. ... 45
Figure 43 - Component 2 (X axis cart) exploded view. ... 46
Figure 44 - Component 3 (X axis mirror mount) exploded view. ... 47
Figure 45 - Component 1 (X axis stepper mount) exploded view... 48
Figure 46 - Threaded rod and nuts transmission assembly of the X axis. ... 48
Figure 47 - Exploded view of the X axis smooth rod clamping. ... 49
Figure 48 - Highlighted Z axis. ... 49
Figure 49 - Beam guiding system. ... 50
Figure 50 - Variable resistance 15º step motor diagram... 52
Figure 51 - Axially magnetised rotor diagram of a hybrid stepper motor. ... 53
Figure 52 - Cross section and detail diagrams of a 1.8º step hybrid stepper motor. ... 53
Figure 53 - HY-TB3DV-M driver board based on the Toshiba TB6560AHQ. ... 54
Figure 54 - The 6-switch DIP packages. ... 55
Figure 55 - Diagram of 1-2phase excitation. ... 57
Figure 56 A diagram of the DB25 male connector pin-out. ... 59
Figure 57 - Mach3 running under Windows XP. ... 60
xiii
Figure 59 - HMI configuration of the Mach3Mill profile. ... 61
Figure 60 - HMI configuration of the Plasma profile. ... 62
Figure 61 - a) Selection of native units warning (mistakenly titled Mach4); b) default units setup. ... 63
Figure 62 - MDI screen of the Plasma profile. ... 63
Figure 63 - Jogging commands fly-out tab position on screen... 64
Figure 64 - ToolPath screen environment. ... 65
Figure 65 - Control circuit wiring diagram. ... 66
Figure 66 - Motor Outputs pin configuration tab. ... 67
Figure 67 - Output Signals pin configuration tab. ... 67
Figure 68 - Input Signals pin configuration tab. ... 68
Figure 69 - Home/SoftLimits configuration window. ... 68
Figure 70 - Motor Tuning and Setup window. ... 69
Figure 71 - Part program and tool path in the G-code display and toolpath windows. ... 70
Figure 72 - List of the available wizards in Mach3. ... 71
Figure 73 - Window of the running Write wizard. ... 72
Figure 74 - Displayed tool path of hello.tap (inverted colours). ... 72
Figure 75 - a) Autodesk TrueView 2015 displaying the DXF file; b) imported design displayed on LazyCam. ... 73
Figure 76 - Mach3 running the generated part program. ... 73
Figure 77 - The diode laser device. ... 74
Figure 78 - Diode laser driver diagram. ... 74
Figure 79 - The assembled prototype. ... 78
Figure 80 - Ablation spot and fumes being caused by the guided beam. ... 78
Figure 81 - Cutting a rectangle out of a sheet of paper. ... 79
Figure 82 - Jogging of the cart for engraving on a wooden block... 79
Figure 83 - Attenuation of the laser radiation by the filtering panels... 80
xv Table of Tables
Table 1 Prototype technical characteristics to aim for. ... 29
Table 2 Stepper motor characteristics. ... 51
Table 3 Driver board characteristics. ... 54
Table 4 Dip switch settings for the excitation/stepping modes. ... 55
Table 5 Dip switch settings for torque or current limiting. ... 56
Table 6 Dip switch settings for decay rate. ... 56
1 1. Introduction
The present document reports the endeavour to develop a didactic laser engraver. The project is essentially the dissertation for obtaining a Master’s degree in Mechanical Engineering, by the Engineering Faculty of the University of Porto (FEUP).
In this first chapter, the project is introduced with an overview of its theme, objectives, and report structure. There is mention of the motive and desired outcome, which are duly used for elaborating the conclusions at the end of this document.
1.1 Motive
Laser cutting and engraving are but two of the many kinds of conventional laser processing applications in manufacturing. The use of laser devices in industrial applications is ever more becoming commonplace, with systems for a wide variety of materials processing and part manufacture, such as surface treatment, cutting, welding, and marking. Moreover, laser based technologies have become important or even dominant in these industrial applications [1, 2]. However, laser cutting and engraving systems have also found a place in the home of entrepreneurs, enthusiasts, artists, and hobbyists who wish to augment their activity. In these cases they are compact, desktop sized machines which would seemingly be well suited for educational purposes in the field of engineering and this is the motive driving the project at hand. There is the need for a low cost solution for a laser engraver, which is to be used in a didactic environment.
Commercially available desktop laser engravers would be plausible candidates to solve this problem, if it were not for a small number of issues. The price of such machines can supposedly drop as low as $100 (around €75), when supplied by dubious vendors, and seemingly more reliable offers can easily reach $10000 (€7500). Professional grade systems are consistently expensive, for example, the Epilog Zing16-30 entry level engraver (see Figure 1.a) costs $8319.69 at the time of writing (€6204.09), while the top of the line model Fusion40-120 (see Figure 1.b) is priced at $45258.26 (€33749.63).
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a
b
Figure 1 - Example laser engravers, a) Epilog Zing desktop laser engraver; b) Epilog Fusion laser engraver and cutter.
Another issue is that it is very hard, if not impossible, to find a desktop laser engraver that does not use a carbon dioxide (CO2)laser tube for engraving. These are laser devices that are
typically rated at 30 W of optical output power, but consumption is much higher. Even though they are among the most efficient type of lasers (usually 15 to 25%), they cannot compare to laser diodes (50% and above). Furthermore, the lifetime of CO2 tubes, before the need for a
refill, is of about 1000 hours, and this factor can only improve to the detriment of their efficiency, by using a gaseous mix of other constituents besides CO2. Lastly, these laser devices
emit infrared radiation, which corresponds to a 10600 nm wavelength. Operating with invisible light poses a potential hazard, since without proper safety implementations and precautions there is a great risk of losing one’s eyesight without so much as a blink.
All of the above discourages the purchase of a commercially available desktop engraver in light of the alternative. Instead, a didactic engraving system using a low powered laser diode is to be developed.
1.2 Objectives
The main objective of this dissertation is the development of a laser engraving system to be used for didactic purposes. It is desired that the system is able to engrave and/or cut soft materials using a low powered diode laser device. As such, some directives for this project’s development may be established:
A prototype needs to be developed;
The prototype must successfully engrave and/or cut soft materials such as wood and paper;
The prototype should be designed to be safe to operate;
The prototype should be designed to allow further development and not be bound by the currently desired processing application.
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The steps needed to meet these requirements also serve as guidelines for designing and developing the system, and they can be summarised thusly:
- Researching and study of laser and laser engraving technologies; - Specifying safety, usability, and technical requirements;
- Discussing a solution for the problem at hand; - Designing and building a prototype;
- Testing the prototype.
1.3 Report structure
The report is comprised of seven chapters. With the goals and development structure in mind, the report is organised in the following manner.
The first chapter is the present one, where an introduction to the project is given, declaring the context, motive and objectives, as well as this very section which overviews the report structure. The second chapter provides the information gathered in order to ascertain the necessary knowledge to fulfil the task. There is firstly an introduction of laser technologies history and basics, followed by a synopsis about diode laser devices. Then, laser cutting and engraving technologies are reviewed, concerning their place in laser processing systems and the aspects of their mechanical configurations and control.
The third chapter contains the project specification, detailing the design principles underlying the whole endeavour and the concept of a laser engraver system. Safety concerns, usability and maintenance issues are investigated and debated in the first section, and following it is the initial idealisation of the laser engraving system, describing features for safe and facilitated use, as well as a consideration of what the machine will be comprised of.
The fourth chapter details the development effort, exposing all the steps taken to design a prototype capable of meeting the objectives. Firstly, the prototype’s first models are presented, depicting the initial attempts created with the help of a CAD software suite. Then, the final iteration of the design is described and lastly the mechanical assembly of the axes of motion is explained.
The fifth chapter is dedicated to presenting the motion control system of the prototype, as well as the driving circuitry of the diode laser device. The driving system of the prototype and the control software used for its operation are described therein, followed by an explanation of the configuration for this project and of processes for part program creation and execution.
The sixth chapter firstly depicts the assembled prototype and the tests carried out to fulfil the objectives.
5 2. Preliminary study
In order to acquire relevant knowledge on laser cutting and engraving machines the fundamental notions about diode laser technologies, as well an overview of laser processing machines, have been researched. This chapter is a synopsis of the information gathered. Basic principles of laser operation were firstly studied, converging on a review of semiconductor laser types and diode laser devices.
Secondly, laser cutting and engraving machines are put into perspective among laser processing machines and are subsequently discussed.
2.1 Study of laser diodes
2.1.1 Laser fundamentals
The word “laser” derives directly from the acronym LASER (Light Amplification by Stimulated Emission of Radiation). The term is generally used to refer to devices which emit an intense and very stable beam of monochromatic, coherent, and collimated electromagnetic radiation. In other words, a laser is a light source, but unlike conventional sources they emit light in one single wavelength, or within a very narrow part of the spectrum [1].
The quantum process of stimulated emission is the basic principle behind laser radiation (see Figure 2). To describe it very simply, it happens when an electron of an atom or molecule finds itself in a higher energy state E2. If a photon with an energy approximately equal to E2 – E1
interacts with this electron by passing by, there is a probability that the latter will be stimulated into decaying to the lower energy state E1. When doing so, its energy may be released in the
form of an extra photon at the exact same wavelength, in exactly the same direction, and with exactly the same phase as the stimulating photon [2].
6
Figure 2 - Stimulated emission illustration diagram.
In 1954 James P. Gordon, Charles H. Townes, and Herbert J. Zeiger developed the first device which made practical use of stimulated emission, called the MASER (Microwave Amplification by Stimulated Emission of Radiation). As the name implies, it emitted microwave radiation. The underlying theory was put forth by the work of Albert Einstein in 1917, when applying Plank’s law of radiation to predict stimulated radiation, and Rudolf W. Ladenburg observed and confirmed the phenomena in 1928. Theodore Maiman created the first laser in 1960, which consisted of a ruby rod surrounded by a helicoidal flash lamp (see Figure 3). The lamp optically pumped the synthetic ruby crystal to generate red radiation at 694 nm [3].
Figure 3 - Schematic of Theodore Maiman’s ruby laser.
Laser devices take advantage of the stimulated emission of radiation through the combination of three elements: a pumping source or pump, a gain medium, and a resonating cavity [3]. In Maiman’s ruby laser, the lamp is the pump, the ruby rod is the gain medium and the resonator is formed by the pair of opposing mirrors.
The pump generates a population inversion in the gain medium, meaning that it creates a greater population of atoms with electrons in a higher energy state than in a lower one through a nonequilibrium process, such as optical or electrical pumping. This sets up the condition for reaching the lasing threshold, for which stimulated emission thence dominates over spontaneous emission in the gain medium.
7
The gain medium, as so far implied, serves as the physical ambient for stimulated emission to occur, consisting of solid, liquid or gaseous matter in which the population inversion is created. The light emitted as a result of stimulated emission is then amplified by the gain medium in a positive feedback loop, achieved by recirculating the light within a resonating cavity, usually consisting of two parallel and opposing mirrors. The output light is let through one of the mirrors, which is partially reflecting.
A very simplified schematic of the working principle of a laser is presented in Figure 4.
Figure 4 - Simplified diagram of the elements of a laser device.
There are many different types of laser devices, but it is possible to distinguish them as [1]:
Gas lasers – lasers in which the gain medium is an electrically excited gas, such as HeNe (helium-neon), HeCd (helium-cadmium), CO2 (carbon dioxide), and Ar+ (argon
ion). The more powerful excimer lasers also use gaseous gain mediums.
Solid-state lasers – based on crystals or glasses that are pumped with discharge lamps or even another type of laser, diode lasers. Examples of gain media are ruby crystals or Nd:YAG (neodymium-doped yttrium aluminium garnet).
Fibre lasers – ion-doped optical glass fibers that can allow high output power, high beam quality, and wavelength-tuneable operation.
Semiconductor lasers – most commonly these are diode lasers. Diode lasers are the subject of the following subsection.
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2.1.2 Diode laser devices
Diode lasers are a semiconductor type of lasers, which generate laser radiation using laser diode chips. Laser diodes essentially consist of a semiconductive p-n junction diode, similarly to light emitting diodes (LEDs). Compared to other laser types they can be distinguished as compact, low powered, and efficient devices. Most commonly they are found in CD or DVD players and recorders, as well as laser pointers, as TO-can package devices (see Figure 5) with an output power not usually greater than 5 mW [6, 7].
a
b
Figure 5 - a) TO-can diode laser package; b) cross section diagram of such a package.
This type of laser relies on the semiconductor chip’s structure for lasing action to occur. In its most basic form (see Figure 6), this structure may be described as two parallel layers of semiconductor material, one being doped n-type material, the other p-type, separated by a thin active region typically measuring 1 µm. In this junction region, light is amplified in a direction parallel to the region’s plane by the chip’s two opposing cleaved faces, forming a resonant cavity [7, 8].
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This type of structure is a homojunction, so called because the p-n junction is made of the same semiconductor material, predominantly gallium-arsenide (GaAs). There are also heterojunctions, or heterostructures, which use multiple layers of different semiconductor materials to form diode junctions. One such example is given by Figure 7, depicting a double heterostructure in which the material composition changes twice in the active region (p-AlGaAs/GaAs/n-AlGaAs) [7, 8].
Figure 7 - Layers forming a double heterostructure.
Single and double heterostructure laser diodes have been developed due to their many benefits compared to homostructure types, including lower losses, lower current requirements, reduced damage, and longer lifetimes. Something that the laser diode structures mentioned so far have in common, as well as all but the last of the structures listed below, is that they are edge emitting structures. As the term implies, edge emitting laser diodes output light through the edge of the active region. In contrast, the more recently developed surface emitting types emit light perpendicularly to the junction plane, as is the case for vertical-cavity surface emission lasers (VCSELs).
The main laser diode structure types are the following [7, 9]:
Homojunction lasers diodes – the simplest structure.
Heterojunction laser diodes – single or double heterostructures are the most common types of structures in laser diodes.
Quantum well laser diodes – the active region is a quantum well, which is a thin layer that can confine (quasi-) particles, typically electrons or holes, in the dimension perpendicular to the layer surface, whereas the movement in the other dimensions is not restricted.
Distributed feedback and distributed Bragg reflector laser diodes – DFB and DBR lasers incorporate a diffractive grating which acts as an optical filter, in order to select a single wavelength to be fed back into the gain medium.
VCSELs – unique for the fact that emission occurs perpendicularly to the active layer; this relatively new type of lasers offer many advantages over edge-emitting types, including greater efficiency, lower threshold currents, and higher beam quality.
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Laser diodes and diode laser are terms which are often used interchangeably, however, the latter designates the semiconductor chip that performs lasing action, while the former refers to complete systems or modules. As such, a diode laser device may be understood as any semiconductor type of laser system which uses laser diodes.
The main considerations for proper handling and operation for preventing damage to a diode laser device may be addressed. These devices can be easily damaged, and their lifetime severely reduced by running at over the specified operating temperature [4]. Thereupon the damaging mechanisms are worthy of note:
- electronic mechanisms: The main cause for catastrophic device failure, often by electrostatic discharge. When working with diode lasers it is important to ground oneself electrically. Electrical spikes and transients also present risk, and may be caused by power surges, lightning strikes, and sudden loss of power. Surge protection can help preventing damage.
- thermal mechanisms: Laser diodes are extremely sensitive to working temperature conditions, the device’s properties and lifetime are heavily influenced by them. A rule of thumb is that for every 1 ºC rise above the working temperature, a laser diode’s lifetime decreases by half. Ambient temperature also affects the performance and may contribute to damaging a device. Thermal as well as power regulation are essential when a diode laser is operating.
Complementary to damaging mechanisms, the most concerning absolute maximum ratings must be considered. The diode laser manufacturer should always specify the maximum power output or drive current, and maximum operating temperature range [4]. Thus:
- maximum power output: This value will indicate the maximum output power that can be achieved with the specified drive current. The maximum drive current must not be exceeded whatsoever. This sensitivity is due to a significant positive feedback when a device is lasing. Overcurrent constitutes a risk of damage to the facets of the laser diode chip and therefore care should be taken when operating at the maximum specified value.
- maximum temperature range: Due to transients in temperature it is advisable to operate a device below the upper limit of this range. The thermal damaging mechanisms are still at play within this range, being so that the higher the operating temperature the less lifetime is expected of a diode laser. Higher operating temperatures also increase the necessary lasing threshold current and may render the specified value useless.
The most relevant criteria when selecting a diode laser device for this project are perhaps the wavelength, threshold current, and operating current, or power output. That being said, two of those specifications beg a few more words.
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Output beam wavelengths of diode lasers range from near infrared (NIR) to ultra violet (UV) light, so a diode laser device emitting within the visible spectrum (wavelengths of approximately 400 to 700 nm) may be most interesting for didactic purposes. Threshold current is the lowest drive current at which lasing action occurs and this is usually represented in a graph such as the one in Figure 8, correlating the light output with the drive current [5].
Figure 8 - Light output vs current graph indicating the laser, or lasing, threshold.
The diode laser’s wavelength is selected and discussed in section 3.2.2, while the threshold current serves to separate between operating and standby modes in a way which is described in section 5.2. Now, the following section is dedicated to the study of laser cutting and engraving machines.
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2.2 Laser processing systems
2.2.1 Laser processing of materials
Industrial applications of laser technologies are varied and widespread, ranging from use in medicine and healthcare to the aerospace field of engineering. Particularly in material processing, laser devices have been applied for their appreciable properties. The fact that a collimated beam of laser light can be focused to achieve extremely high irradiance at the surface of a workpiece, producing very large heating rates in the affected volume, means that lasers can be used for precision processing with small heat-affected zones [6].
Some more advantages of laser processing over conventional processing technologies can be listed [6]:
Absence of mechanical contact with the workpiece, meaning there are no cutting forces nor tool wear;
Ability to work with refractory or hard, brittle materials with little difficulty;
Extremely small welds may be achieved;
Inaccessible areas or even encapsulated materials can be reached with the laser beam;
Easy and fast fixturing, speedy setup times, and no need for vacuuming lead to rapid throughput and prototyping;
High quality cutting, no need for finishing operations.
Material processing applications for laser technologies include welding, cutting, drilling, marking and scribing. For each application different types of lasers are used, with different wavelengths and operating modes. The two dominant kinds of laser technologies used in materials processing are CO2 and Nd:YAG lasers. Other commonly used lasers are ruby, argon,
and excimer lasers [6].
Laser processing usually involves removing material from a workpiece through the following mechanisms: melting, vaporisation, and chemical degradation [1, 2]. The thermal energy absorbed by the work surface when a high energy laser beam is focussed upon it leads to the transformation of the affected area into molten, vaporised, or chemically changed state. The schematic in Figure 9 classifies laser processing applications according to these mechanisms and processes that involve no change of phase.
13 Figure 9 - Classification of some laser processing applications by phase change mechanisms.
A more useful classification from an applications’ point of view is to group them into broad definitions of their kind of material processing. In other words, laser processing may be classified as forming, joining, machining, or surface engineering [6]. Figure 10, then, organises this information in a more pleasant way.
Figure 10 - Broader classification of some laser processing applications.
Among these kinds of laser processing applications, focus is given to machining applications in this report. More specifically, the next subsection will follow up with an overview of cutting and engraving1 applications.
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2.2.2 Systems for laser cutting and engraving
Laser cutting and/or engraving machines are essentially composed of five elements: the laser source, the laser beam delivery or guiding system, the cutting or engraving head, the position control system, and the power supply unit [7]. The laser source can be understood as the unit which produces and controls the optimal functioning parameters for the laser beam. The laser beam delivery system includes the accessories needed to provide beam guidance, such as fixed or articulated robotic arms and adjustable guiding mirrors, while the cutting head provides beam focusing. The elements that control and drive the position of the beam relative to the workpiece allow meaningful work to be done and they are the control unit and the mechanical axes of motion. Figure 11 shows a schematic representation of these elements.
Figure 11 – Laser processing machine components: 1-laser source; 2-laser beam delivery; 3-cutting head; 4-work table axis of motion; 5-control unit; 6-power supply unit.
Laser cutting and/or engraving machines usually feature one of four main configurations: fixed optics, flying optics, hybrid, or scanned laser projection systems [6].
On fixed optics systems (see Figure 12), the workpiece’s position is controlled by moving axes under the laser head, which is stationary or moves solely in the vertical Z direction. This setup favours a good optical conditioning of the laser beam, but it compromises agility of the machine, given the great inertia of the moving components, as well as size. An independent motion for focusing the beam is necessary for height adjustment and workpiece irregularity compensation. The vertical motion may be performed by just the focusing lens or laser source, or even the entire laser head containing both laser source and beam delivery systems, considering the configuration is application dependent.
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Overall, fixed optics is best suited for very precise work on small or medium sized workpieces, because of the great stability provided by the lack of vibrations affecting the laser beam and the issue with size.
Figure 12 - Fixed optics configuration schematic.
Flying optics (see Figure 13) consists in controlling the position of the laser head, as opposed to the workpiece, making use of adjustable mirrors for beam delivery. The workpiece is affixed to a stationary workbed, while the axes of motion produce the cutting path by moving either a laser head, laser focusing lens, or even the entire laser delivery system. Workpiece size, rather than range of movement, determines these systems’ footprint, which leads to greater simplicity in fixture and accessibility designs. Also, the workpiece’s weight does not affect accuracy, as motion is applied to the optics, nor does its variation influence smoothness of motion.
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The fact that flying optics systems are suitable for work over large dimensions, both in area and thickness, leads to the need for powerful laser sources. In these cases it is impractical to move the laser source, as it would be too heavy. Consequently, the laser source needs to be external to the processing machine, which implies it should remain stationary in a compartment all of its own. This implicates that the distance from the laser output to surface of the workpiece will not remain constant along the cutting path, leading to non-uniform cutting performance at different points due to beam length variation.
There are also hybrid combinations of fixed and flying optics systems. Truly, these systems combine the advantages and disadvantages of those systems. Whichever the combination, in hybrid systems neither the optics nor the workbed is stationary. One axis may move the laser head for positioning in one Cartesian coordinate, while the orthogonal axis moves the workpiece (see Figure 11).
Scanned laser projection, or indexed beam steering, involves sweeping the laser beam by means of two rotation driven mirrors, one for X coordinate position, the other for the Y coordinate (see
Figure 14). The independent rotation of each mirror allows for the orthogonal movement of the beam. Given that the laser source is usually low powered and therefore lightweight, it is usually mounted on the laser head, placed directly above the workpiece. In this configuration, both elements are static, so there are little to no issues related to mechanical inertia. However, this kind of system is only feasible for small processing areas due to issues with focal length, which varies with the mirrors’ angle away from centre. Systems with scanned laser projection are typically used in production lines for part identification.
Figure 14 - Scanned projection configuration schematic.
Being motion controlled systems, laser cutters and engravers may use computer numerical control (CNC) for their operation. In essence, the CNC controller is the commanding element of the machine, in charge not only of controlling the motion and position of the system, but also of logic operations. A CNC controller is comprised of three elements: the Man, or Human, Machine Interface (MMI/HMI), the Numerical Control Kernel (NCK), and the Programmable Logic Control (PLC) [8].
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The HMI must serve as the usability interface between an operator and a machine tool, generally featuring five different functions:
Operation functions – those that allow operating the machine; machine status, position, feed rate, spindle speed and other operating data is displayed, jogging and manual data input (MDI) are also provided;
Parameter-setting functions – those that allow setting of machine parameters (machine regulation, driving systems, spindle, tool offset, work coordinates, and safety boundary), program parameters (set during editing), and customisation parameters;
Program-editing functions – those that allow editing and modifying part programs (essentially G-code programming);
Monitoring and alarm functions – those that provide overall monitoring information;
Service or utility functions – those that don’t fit any of the above, but provide useful features.
The NCK, being the key unit of a CNC system, is tasked with the interpretation of instructions and commanding the driving system accordingly. To that end, it features the following functions:
Interpreter – reads and interprets the ASCII blocks in a part program, storing the resulting data in memory for use by the interpolator;
Interpolator – sequentially reads the data, calculating position and velocity per unit of time for each axis and storing the result in a first-in-first-out (FIFO) buffer for use by the acceleration/deceleration controller;
Acceleration/deceleration control – two methods exist to avoid mechanical vibration and shock at beginning and end of part movement: the data generated by the interpolator is filtered by executing acceleration/deceleration (A/D) control before executing position control, or A/D control is executed before both interpolation and position control.
Position control – position control is executed in a constant time interval based on the data transmitted by the A/D controller.
The PLC is responsible for sequential and logic control, such as turning coolant on or off, tool changing, I/O signal processing, and overall control over the machine’s behaviour, save for commanding the axes. It can be defined as a controller, consisting of a CPU and memories, that can edit, execute, and modify PLC programs. It is comprised of the following elements:
Programming tool – permits the editing of a program and its loading to the CPU;
Input unit – receives binary ON/OFF signals from various components like sensors and switches, and converts them into signals the CPU can interpret;
Output unit – sends output binary ON/OFF signals;
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DATA memory – stores executable program such as the operating system;
CPU unit – tasked with performing logic calculations.
All three modules can be seamlessly executed by modern day PC platforms using soft-CNC solutions. One such software CNC controller was selected for use in this project and is discussed in section 5.1.2 of chapter 5. The project specification is the subject of the next chapter, chapter 3.
This concludes the synopsis of the information gathered for a fundamental understanding of laser cutting and engraving systems.
19 3. Project specification
In the context of this project, a working prototype should be designed and assembled. Its concept takes form from the study of laser cutting and engraving technologies and the desire to fulfil the objectives. In the light of this, the current chapter is dedicated to defining project requirements, including safety concerns, usability, and maintainability of the machine.
First and foremost, safety concerns regarding the operation of laser devices in general are discussed. This is shortly followed by a research of safety requirements and measures for a laser engraver, for which international standards ISO 11553-1 and IEC 60825-1 have been taken into consideration.
Secondly, since it is meant for a didactic system to be developed, features that facilitate usability and maintenance of the machine are idealised. The chapter then ends elaborating on a concept for the prototype, as well as the technical specifications to aim for in its development.
3.1 Requirements
3.1.1 Safety concerns
Several hazards are associated with the operation, maintenance, and even the mere presence of a laser device. Primarily, the high radiance property of laser beams, i.e. their high power density and directionality, is the most concerning factor associated with eye hazards [3].
Damage to the eye’s retina can occur even with lasers whose output power is of a few miliwatts, since focusing by the eye lens can produce power densities in the order of kilowatts per square centimetre, on a dot 10 to 20 µm in diameter. To stress this point, it can be said that this is because the laser power density may be increased by a factor of 105 once it reaches the retina. Making matters worse is the possibility of deflected or diffuse reflection of the laser beam, which for powerful enough lasers can just as easily cause significant damage.
Retinal damage is a risk with wavelengths varying between 400 to 1400 nm, which corresponds to the whole visible spectrum (400 to 700 nm) and the near infrared region (700 to 1400 nm). Corneal damage happens with both far infrared (above 1400 nm) and ultraviolet (if shorter than
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315 nm) laser radiation. Skin damage is also a risk in the case of sufficiently long exposure to the laser beam.
In addition, many laser devices rely on potentially dangerous equipment for their operation, giving rise to, chiefly, electrical hazards. Some examples are high voltage power supplies, capacitors charged to lethal voltages, and accessories such as Pockels cell Q-switches, modulators, and optical gates, all of which operate at high voltages as well.
To further instil an appropriate cautious attitude2 upon the reader, it has been stated that “there are very few people working in the laser filed who have not had a colleague injured or killed while using a laser” [3]. Clearly, studying the appropriate safety requirements and measures is of the utmost importance. In this case, it is essential to research safety issues concerning a laser engraver.
The general safety requirements for a laser processing machine3 are within the scope of ISO
11553-1, which is why it is sensible to take the document as a guide. This implicates that a great deal of other normative documents are to be consulted as reference, but the focus of this section is to bring to light the essential safety concerns regarding the operation and maintenance of the laser engraver.
The ISO 11553-1 standard lists hazards that are inherent to laser processing machines and those generated by external effects.
Inherent hazards are:
- mechanical, electrical, thermal, vibration and radiation hazards, and those generated by materials and substances (constituents of the machine or processed) and by neglecting ergonomic principles in machine design.
External effects that generate hazards are:
- temperature, humidity, external shock/vibration, vapours, dust or gases from the
environment, electromagnetic/radio frequency interference, source voltage
interruption/fluctuation, and insufficient hardware/software compatibility and integrity.
The general requirements to ensure safety of laser processing machines are defined thusly: - hazard identification and analysis,
- implementation of safety measures,
- certification and verification of the safety measures, - provision of appropriate information for the user.
This makes for a well-defined set of guidelines that are perfectly suitable to the design of a safe machine. In the project specification and development, efforts are made to comply with the standard’s required implementation of safety measures.
2 Fear.
3 Laser processing machines are defined, in ISO 11553-1, as “machines in which (an) embedded laser(s) provide(s)
sufficient energy/power to melt, evaporate, or cause phase transition in at least a part of the workpiece, and which has the functional and safety completeness to be ready-to-use”
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Aside from the general guidelines provided by ISO 11553-1, IEC 60825-1 specifies requirements for safety features such as protective housing, warning signs and other protections from hazards, as well as a classification system for laser products (see Annex A). The safety features depend on the class assigned to the laser product, and a total of 11 clearly defined features are described in IEC 60825-1:
- protective housing, for preventing human access to laser radiation in excess of the accessible emission limit (AEL) for Class 1 given in table 1 of IEC 60825-1 (applicable to all laser products);
- access panels and safety interlocks, for maintenance tasks which require removal or displacement of the protective housing, giving access to hazardous laser radiation levels; - remote interlock connector, for limiting accessible radiation to the AEL for Class 1 when
the connector’s terminals are open-circuited (applicable to Class 3B and Class 4 laser products);
- manual reset, for enabling the resumption of Class 4 laser radiation emission after an interruption by the previous feature (applicable to Class 4 laser products);
- key control, to prevent access in the absence of a master key (applicable to Class 3B and Class 4 laser products);
- laser radiation emission warning, for persons in the vicinity of the laser product, when it is switched on or in case capacitor banks are being charged or have not been discharged (applicable to Class 3R laser systems in the wavelength below 400 nm and above 700 nm, as well as Class 3B and Class 4 laser products);
- beam stop or attenuator, provides means of beam attenuation for preventing human access to laser radiation in excess of the AEL for Class 1 (applicable to Class 3B and Class 4 laser products);
- controls, for providing controls located so that adjustment and operation do not require exposure to laser radiation equivalent to Class 3R, Class 3B or Class 4 (applicable to all laser products);
- viewing optics, must provide sufficient attenuation to prevent access to laser radiation in excess of the AEL for Class 1M (applicable to all laser products);
- scanning safeguard (applicable to laser products intended to emit scanned radiation); - “walk-in” access (applicable to all laser products allowing “walk-in” access).
Labelling of laser products is standardised by IEC 60825-1, requiring that labels are durable, permanently affixed, legible, and clearly visible during operation, maintenance or service, according to their purpose. They must be placed so that they can be read without the implication of human exposure to laser radiation in excess of the AEL for Class 1. It is also established that text borders and symbols on labelling must be black on a yellow background for all laser classes except for Class 1, for which it is not mandatory (see Figure 15). Complementary to the warning sign, an explanatory label must be affixed, containing the words conforming to the assigned classification (see Figure 16). Figure 17 depicts a labelling example for a Class 2 laser product.
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Figure 15 - Warning label.
23 Figure 17 - Class 2 laser product warning and explanatory labels.
A laser engraver would be considered safe should it comply with all of the above requirements. Depending on the assigned classification, it may not be necessary to design all safety features listed in IEC 60825-1, as it is specified. However, if a prototype is to be developed for this project, determining its classification as a laser processing machine is beyond the self-imposed responsibilities, at least in the context of the present dissertation. As such, only a warning label is required, while the explanatory label is not. In case the latter is affixed to the prototype it shall pertain only to the selected laser device contained within.
3.1.2 Usability, maintenance, and safety features
Without forsaking the previous concerns, some principles for the system’s usability and maintenance are established in this short section.
Summarily, the subjects being considered in this section are:
The didactic potential and requirements
Control interface solution
Upgradeability
Maintenance safety and restrictions
Prototype safety requirements
The laser engraver is meant to be used fundamentally as a didactic tool. The meaning of this is twofold: the machine may be used in a didactic environment as a tool for projects of various disciplines, and it may also be used for machine tool and CNC training at a basic level. As a didactic system, it is not required that the prototype integrates autonomous teaching exercises that are capable of dynamically training the user. Rather, the laser engraver should itself integrate in didactic activities and in that sense be a passive element in teaching. Nonetheless, operating the machine ought to be a swift process, calling for a consideration of the interaction between machine and user.
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It would be strenuous to develop a fully operational control system in the context of this dissertation, as it would involve creating, testing, and validating control hardware and software from the ground up, so an existing solution would preferably be considered. Furthermore, the use of a tried and true control system, with well documented support, should facilitate usability. Therefore, a CNC software package, which is the subject of section 5.1.2, has been selected to implement the motion control system of the prototype.
Future undertakings could involve upgrading or requalification of the prototype, both of its control system and of its application. Desktop laser engravers commonly have only 2 axes of motion in a flying optics arrangement, which makes it difficult to implement other manufacturing capabilities, such as 3D-printing. Designing a system with 3 Cartesian axes of motion grants more flexibility and potential for future work on the prototype. In section 3.2.1, the concept takes this matter into consideration and suggestions of future work on the machine are presented in chapter 7.
On another note, the maintainability of the machine must be accounted for.
Firstly, maintenance should be carried out with proper knowledge of the machine’s design. The machine will draw power from a mains line, potentially generating electrical hazards even when turned off. All the hardware needs to be handled with care, especially the laser device. Repairing wired connections requires close attention, since incorrect rewiring could lead to malfunction, perilous function or even a fire hazard. All of this means that any repair and maintenance effort should be done only after a careful assessment of the issue that needs to be resolved and the required repairing tools and procedures for doing so safely.
Secondly, access to hardware components pertaining to laser powering and control should not be easily granted. As specified in the previous section, maintenance tasks which require displacement of the protective housing should be safeguarded by access panels and safety interlocks. However, the prototype implementation can be safely maintained requiring only the disassembly of the protective housing in a powered down state.
The minimum safety requirements and implementations for the laser engraver are as follows: - Protective housing
- Laser radiation filtering panels - Emergency-stop button
- Open/closed access sensor switch - Warning label
The basic principles for safe usability and maintenance of the laser engraver are hereby established. The next section, section 3.2, follows up with the concept and design considerations of a prototype.
25 3.2 Prototype definition
3.2.1 Concept and design considerations
This section introduces a concept for the laser engraver prototype. The main points to take from these paragraphs are the following:
The prototype is, ideally, a “black-box”
The protective housing becomes the supporting structure of the prototype’s assembly
Radiation filtering panels and other important safety features are considered
Size constraint is suggested and the axes of motion configuration is determined
Beam guidance and workbed levelling solutions are approached
When using the laser engraver, the user should only need to ready a workpiece for engraving or cutting, execute the operation, and, at the end of it, retrieve the finished product. In other words, the machine is intended to be, ideally, a “black box”. This essentially means that all elements which can reveal the working principle of the prototype would preferably be hidden from view. In practical terms, the ease of use and work area accessibility must not be compromised by this intention, but since a protective housing is required to begin with, it is admissible to at least conceal all the electronic hardware inside an opaque barrier. To reiterate, easy accessibility to the work area must still be granted, and proper filtering of the laser radiation must still be ensured. Figure 18 depicts a sketch inspired by this concept.
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This sketch encompasses the intention of separating the components of the prototype into to two distinct sections, a characteristic that is explained further down. The filtering panels are for protection against laser radiation above the AEL for Class 1, as specified. There are laser filtering acrylic panels available that protect against laser radiation in a wide range of wavelengths and could be used in the protective housing. Alternatively, adhesive polarising film may be affixed to non-filtering panels. Whichever solution is chosen the selected laser diode’s emitted wavelengths must be taken into account.
To frame the panels, slotted bars may be used so that the panels can be easily slid into place. This frame is not only meant to serve as the protective housing, but also as the supporting structure for the mechanical assembly. As such, it needs to be the load carrying and supporting member of the machine, requiring sufficient rigidity for this purpose. Using steel bars, for example, would greatly increase the mass of the structure leading to a more aggravating issue with vibrations, while framing with aluminium bars makes for a lighter structure without significantly sacrificing rigidity. In section 4.1, the selected profiles are mentioned when the first design drafts are described.
For granting accessibility safely and easily, a hinged door occurs as a satisfying solution (also featured in the sketch in Figure 18). Safety should be ensured by designing it so that, when open, it disables machine operation entirely, leaving no room for accidental or unwarranted exposure to operating levels of laser radiation. This can be achieved by affixing a magnet to the door that will be detected by a Hall-effect sensor switch (see Figure 19.a). Another essential safety feature is to have in place an emergency-stop button (see Figure 19.b) which, similarly to the opening of the door, brings operation to a halt when activated. These elements integrate the control system of the prototype, which is the subject of section 5.1.3.
a
b
Figure 19 - a) hall-effect sensor example with working principle schematic; b) emergency stop button example.
The prototype ought not to be much bigger than most hobbyist-oriented laser engravers, for which reason the work area should have conservative dimensions. A sensible size for the work area is 210x297 mm, for example, which are the dimensions of the standard A4 paper size. This will allow the machine to be relatively compact, which helps keeping costs low and can also be considered a desirable trait for a didactic system.
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Currently, an axes of motion configuration should be chosen. Although a scanned projection system would be the most compact solution, it does not offer as much flexibility for different applications as fixed or flying optics systems. For this reason and the aforementioned desire to allow upgradeability and requalification potential, only fixed or flying optics, or a hybrid configuration thereof, are considered.
A fixed optics configuration translates to a bigger form factor for the same work area as a flying optics one. Both solutions are faulted with cascading error of an axis over to the other, since they are not independent, due to squareness and straightness errors. A hybrid combination (see Figure 20), with independent X and Y axes of motion, can compensate for this issue while partly sacrificing the smaller size if it were a full flying optics system. For a work area equal to the A4 paper size, the most ergonomic arrangement for this solution is seen on figure hybrid gantry. The flying optics move on the X axis, travelling over the greater side of the work area, while the workbed moves on the Y to travel the shortest side.
Figure 20 - Hybrid gantry configuration sketch indicating the axes of motion.
By observing Figure 18 once again, it may be noticed that there is a section covered by black panels. This section is intended for holding the laser device stationary, as well as all the necessary power and control circuitry. It remains stationary mainly due to its temperature regulation needs, which call for cooling with a fan and heat sink. In other words, the weight and size of the cooling elements forbid the direct manipulation of the laser unit in a reasonable way, considering the chosen mechanical configuration. Mounting it on a separated fixed position might also allow for a facilitated maintenance approach, as the laser unit can be repaired or replaced without interfering with the axes of motion assembly.
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Placing the laser device in a stationary position immediately creates the need for a beam delivery system. To solve this, the beam may be guided by adjustable front surface mirrors. These mirrors are positioned so that they are parallel in pairs and angled at 45º relatively to the respective axis of motion, in order to bend the beam in the correct direction (see Figure 21). A focusing lens with fixed focal distance is lastly used to concentrate the beam and in this way cause ablation on the workpiece.
Figure 21 - Mirror guiding schematic.
Focus height is adjusted by a Z axis to accommodate for different workpiece sizes, making it possible to engrave on either a single sheet of paper or on a book’s cover, for example.
In the proposed gantry configuration the Z axis transports the X axis, but because the former is not meant to move while engraving takes place, the issue of cascading error between these axes is not cause for concern. Most importantly, however, this third axis is designed to further indulge the desire for the prototype’s upgradeability.
Given the expectedly low engraving speeds involved, the workbed features no fixture for holding a workpiece. It adds to this that the workbed moves solely in the Y direction, implying that a workpiece would only need fixturing to prevent sliding “back and forth”. The absence of mechanical contact with the workpiece during engraving further lessens the need for robust fixturing.
On another note, the work bed should have as little contact with the workpiece as possible to avoid acting as a heat sink for it, as well as diminish interaction with the laser beam. This might complicate the solution for holding the workpiece on the workbed, if not deny it completely, but for the reasons stated before this is not considered a pressing issue. Furthermore, the workbed’s orientation should be adjustable, to provide yet another means of ensuring perpendicularity of the beam. This may be achieved by mounting the work bed on 3 spring loaded screws.
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The implementation of these basic design considerations as a working prototype is the focus of chapters 4. Before this, the next section quantifies the desired technical characteristics for the prototype.
3.2.2 Technical characteristics
Now it would be suitable to specify some intended technical specifications. As such, this section proposes the desired capabilities of the laser engraver.
The following, then, is a summary of the essential characteristics, displaced in Table 1.
Table 1 Prototype technical characteristics to aim for.
PROCESSING AREA 210x297 mm (A4 paper size)
WORK VOLUME HEIGHT 200 mm
PROCESSING ABILITIES Engraving and cutting
MATERIAL ENGRAVING CAPABILITY Wood
MATERIAL CUTTING CAPABILITY Paper, cardboard
RESOLUTION (of each axis) 0.025 mm per step pulse (~1000 dots per inch)
LASER WAVELENGTH 445 nm (blue)
LASER OPERATION Continuous mode
By now, having been considered in previous sections, the first few items come as no surprise. Work area size has been discussed, and the greatest admissible height of a workpiece does not define the material thickness that the machine can process, but rather the maximum permissible distance between the laser beam’s focal point and the workbed’s surface.
The material processing capabilities refer to what sort of materials the machine should be able to engrave and/or cut. Depending on material thickness and absorption, the machine could even be able to cut wood and acrylic, but this shall not be required of it. Simply stating “wood” and “paper” might leave room for doubt over what kind of wood or paper the prototype can or cannot process, but determining which materials the prototype would fail to process is beyond the scope of this dissertation. Hence, when the prototype is tested it will suffice to see engraving results on a single, whichever kind of wood and cutting success on a small variety of paper sheets.
The axes’ resolution is expressed in mm per step pulse, defining the total travelled distance of an axis for one step pulse received by the driving motor. Desktop laser engravers usually feature a user adjustable resolution range, typically from 100 to 1000 dpi. The upper limit is used as the reference value, the reasoning being that, if one inch corresponds to approximately 25 mm, then each “dot” or minimum processed distance measures 0.025 mm.
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As for the laser’s characteristics, the output wavelength has been selected to be 445 nm, which is the blue colour in the visible spectrum. Within the spectrum the blue colour is in the lower wavelength limit and therefore is a more “energetic” colour than green and red, for example. Also, the shorter wavelength allows a wider beam to be focused down to a spot more easily than in the case of those other colours. Therefore, the surface coated mirrors should be less affected by the laser beam, while high irradiance can still be achieved in the focused spot. The diode laser device is to be driven by a constant current source for a continuous operating mode. Nevertheless, a working current level for material processing cannot be incessantly provided for non-continuous engraving or cutting paths. This is considered when illustrating the driver in section 5.2 of chapter 5.
In the chapter following the current one, chapter 4, the model of the prototype is presented, with focus on the mechanical assembly.
31 4. Model and mechanical assembly of the prototype
In this chapter the efforts made for designing the prototype are documented. The initial versions, based on the proposed solution, and how they are improved and/or discarded throughout the design process are the subject of the first section. The resulting final stage of the design is then illustrated by a CAD model, followed by a description of the mechanical assembly.
4.1 Initial modelling stages of the design
The prototype is the final iteration of a number of design stages. To achieve the end result other versions had been previously sketched, which are worthy of mention. Hence, the present section will provide insight over the initial design stages.
The model was created with the help of Autodesk’s Inventor Professional 2014, which is a computer aided design (CAD) software suite for 3D modelling.
It has been determined that the machine should have a protective housing, so while it may seem an unlikely starting point for the design, it settles the matter of what the machine could look like. The very first model of the prototype designed with this tool merely stands as a study of the machine’s overall size and compartmentalisation, featuring no functional assembly, as seen on Figure 22.
